In a strainer add 2 tbsp of yellow and 2 tbsp black mustard seeds (50:50), wash under running water, and grind with one green chilli to a smooth paste by using your food processor. Or else, you can use ready to use mustard powder (Sunrise brand). If cooking with the mustard seeds powder, use 2 tbsp of it and follow the package instructions for dissolving.

Wash and cut eggplant vertically into wedges.

In a bowl whisk the yogurt. Add mustard paste, turmeric powder, red chilli powder, and salt. Mix well and keep aside.

Over medium heat add oil in a deep frying pan/kadai. Add eggplant wedges and sauté until golden in colour. Remove from the heat and keep aside.

Add 1½ tbsp oil to the kadai and temper with nigella seeds. Next to add sliced onions and sauté over medium heat until light brown. Lower the heat, add 1/2 cup of water into this. Now add the yogurt mixture and green chillies, mix gently. 

Cover and cook over low heat for 10-12 minutes or until the curry thickens. Now add the eggplant slices and sugar into the curry, mix slowly, and cook uncovered for 5-6 minutes. Remove from the heat.

Drizzle mustard oil on top and garnish with chopped coriander leaves. Serve hot as a side with rice & dal, or have it with roti/paratha/phulka, etc. Enjoy!